JP3348365B2 - Hot-rolled high-strength steel sheet for processing having excellent heat-softening property and excellent fatigue properties, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties and intended for use in automobiles, industrial machines and the like, and a method for producing the same.

[0002]

2. Description of the Related Art There is an increasing demand for high-strength steel sheets mainly for the purpose of reducing the weight of steel sheets for automobiles and ensuring safety in collisions. However, even for high-strength steel sheets, the requirements for workability are severe, and steel-strength steel sheets having excellent workability are desired. Lighter weight due to higher strength (reduced thickness)
Insufficient fatigue strength has become apparent, and there is a strong demand for high-strength steel sheets having not only excellent workability but also excellent fatigue strength. Further, welding is frequently used as a vehicle body assembling means, and it is necessary to have heat softening resistance. That is, it is required to simultaneously satisfy three properties of workability, fatigue properties, and heat softening properties.

Hitherto, Nb-added steel (mainly composed of ferrite and bainite) has been mainly used for high-strength members (for example, wheel rims) requiring heat-softening resistance.
However, the Nb-added steel is inferior in workability and may cause a problem during wheel rim forming. For example, necking occurs at the time of pipe expansion due to insufficient uniform elongation, and dimensional defects in the winding process due to an excessive yield ratio. On the other hand, steel sheets having excellent workability and fatigue properties include low-temperature steels represented by DP steel (mainly composed of ferrite and martensite) and γ steel (mainly composed of ferrite, bainite and retained austenite). Steel utilizing transformation products is known, but heat softening is large because the low-temperature transformation products are tempered. That is, in the prior art, it is the actual situation that the three properties which are sufficiently satisfactory at the same time as the workability, the fatigue property and the heat softening property are not obtained.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot-rolled high-strength steel sheet for processing having excellent fatigue properties and heat-resistant softening properties, and a method for producing the same.

[0005]

Means for Solving the Problems In order to achieve the above object, the present invention has the following constitution. (1) C: 0.03 to 0.20% by weight ratio, A
l: 1.0 to 5.0%, Mn: 0.5 to 3.5%,
Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01% Contains the balance of Fe and unavoidable components, is composed of a first phase of ferrite and a second phase, has a ferrite space factor of 50% or more, and has a microstructure of ferrite and the second phase. Vickers hardness ratio (second phase hardness / ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50.

(2) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01% 800 ° C. using a steel slab containing the balance Fe and unavoidable components
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C.

(3) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Ca: 0.0005-
0.01% or REM: 0.005 to 0.05% Contains the balance Fe and unavoidable components, is composed of ferrite as a first phase and a second phase as a microstructure, and has a ferrite space factor of 50% or more. The ferrite and the second phase have a micro Vickers hardness ratio (second phase hardness / ferrite hardness) of 1.5 or less, and have tensile strength (TS) ≧ 400 MPa as a characteristic; Tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50.

(4) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Ca: 0.0005-
0.01% or REM: 0.005 to 0.05% 800 ° C. using a steel slab containing the balance Fe and unavoidable components
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C.

(5) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Nb: 0.005
0.050% Contains the balance of Fe and unavoidable components, is composed of a ferrite and a second phase as a first phase having a microstructure, a ferrite space factor of 50% or more, and a micro Vickers hardness of the ferrite and the second phase. The ratio (second phase hardness / ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50.

(6) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Nb: 0.005
Using a slab containing 0.050% balance Fe and unavoidable components, 800 ° C
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-softening resistance and excellent fatigue properties, wherein hot-rolling is performed at a temperature of from 1000 to 1000 ° C and winding is performed at a temperature of from 550 to 700 ° C.

(7) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Nb: 0.005
0.050%, Ca: 0.0005 to 0.01% or REM: 0.005 to 0.05% The balance includes Fe and unavoidable components, and as a microstructure, the first phase is ferrite and the second phase. The ferrite and the second phase have a micro Vickers hardness ratio (second phase hardness / ferrite hardness) of 1.5 or less, and have a tensile strength ( TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50.

(8) C: 0.03 to 0.20 in weight ratio
%, Al: 1.0-5.0%, Mn: 0.5-3.
5%, Si ≦ 5.0%, P ≦ 0.05%,
S ≦ 0.01%, Nb: 0.005
0.050%, Ca: 0.0005 to 0.01% or REM: 0.005 to 0.05% Using a steel slab containing the balance Fe and unavoidable components, 800 ° C.
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C.

[0013]

The present inventors have conducted various experimental studies and found that A
1) A second phase (pearlite, bainite, martensite, cementite, residual austenite, and the like) having a small difference in hardness from ferrite, which is a hard first phase to which a large amount of l is added (preferably 150 or more in terms of micro Vickers hardness). By dispersing the tempered structure of the present invention, it has been found that the problems of the prior art can be solved, and that the three properties of workability, fatigue properties and heat softening can be simultaneously achieved, and the present invention has been made. The gist is described below (hereinafter, ferrite as the first phase is referred to as ferrite).

First, the steel sheet microstructure of the present invention will be described in detail. The microstructure of the steel sheet is composed of ferrite and the second phase. The ferrite space factor is 50% or more, and the ratio of micro Vickers hardness between ferrite and the second phase (second phase hardness / ferrite hardness) is 1.5 or less. When the space factor of ferrite is less than 50% or the micro Vickers hardness ratio of ferrite and second phase (second phase hardness / ferrite hardness) exceeds 1.5, the effect of the second phase on the properties of the steel sheet is reduced. The adverse effect increases, making it impossible to combine workability, fatigue properties, and heat-softening resistance. Preferably, the ferrite space factor is 70% or more.

The second phase may be any one or more of pearlite, bainite, martensite, cementite, retained austenite and their tempered structures, and may have a micro Vickers hardness ratio (second phase). Hardness/
(Ferrite hardness) ≦ 1.5. That is, the second phase designation is not limited. Further, even if the micro Vickers hardness ratio between the ferrite and the second phase (second phase hardness / ferrite hardness) is more than 1.5, if the space factor is 5% or less, the effect on the steel sheet properties can be reduced. Since the adverse effects of the two phases are small, they may be contained.

Next, the regulated values of the chemical components and the reasons for the restrictions will be described (hereinafter,% means weight%). C is 0.0
3 to 0.20%. In order to increase the ferrite space factor, it is preferable that C is small. However, if it is less than 0.03%, the Ar3 transformation point rises, and it becomes difficult to secure the temperature during hot rolling. Since the material deteriorates, the lower limit is made 0.03%. Further, from the viewpoint of spot weldability, the upper limit of addition is set to 0.20%. Preferably it is 0.10% or less.

Al content is set to 1.0 to 5.0%. In order to promote ferrite transformation, harden ferrite, and reduce the micro Vickers hardness ratio between ferrite and the second phase, A
Although l is preferably as large as possible, if it exceeds 5.0%, cracks are likely to occur during hot rolling, so the upper limit is 5.0%. If less than 1.0%, ferrite transformation is promoted,
Since the effect of hardening the ferrite, reducing the micro Vickers hardness ratio of the ferrite and the second phase, and ensuring sufficient workability, fatigue properties, and heat softening resistance cannot be sufficiently obtained, the lower limit is 1.0%. Preferably, the lower limit is 1.5%. Also,
Since Si also has the same action, it may be added up to 5.0%, but it is preferable not to add it from the viewpoint of surface properties.

Mn is 0.5-3.5%. 0.5%
If it is less than 0.5%, the Ar3 transformation point rises, and it becomes difficult to secure the temperature during hot rolling, and the material deteriorates in the portion where the Ar3 transformation point cannot be secured. Therefore, the lower limit is 0.5%. If the Mn content exceeds 3.5%, the ferrite transformation is remarkably suppressed, so the upper limit of the Mn content is set to 3.5% or less. P is 2
From the viewpoints of the secondary workability, toughness, spot weldability, and recycling, the upper limit is set to 0.05%. If these requirements are not strict, they may be added in excess of 0.05%. Further, from the viewpoint of obtaining beautiful surface properties, 0.01
% Or more is preferable.

The upper limit of S is set to 0.01% in order to prevent the stretch flangeability (hole expansion ratio) from deteriorating due to sulfide-based inclusions. In order to further improve the hole expansion ratio by controlling the shape of the sulfide-based inclusions (spheroidizing), 0.0005 to 0.01% of Ca may be added. Also, RE
M may be added in an amount of 0.005 to 0.05% for the same reason. Nb contributes to microstructural refinement, exhibits excellent low-temperature toughness, and contributes to securing strength. In order to exert its effect sufficiently, the lower limit of addition is 0.005%.
That is all. However, even if it is added excessively, the above-mentioned effect is saturated and the workability is rather deteriorated.

The above is the main reason for adding the main components of the present invention. Ti, Cr, Cu, N
One, two or more of i, V, B, and Mo may be added. However, if the total amount exceeds 0.2%, it becomes difficult to obtain the microstructure of the present invention and the cost increases, so the upper limit is made 0.2%.

Further, from the viewpoint of how to achieve the above-mentioned microstructure, the values of the rolling regulation and the winding regulation and the reasons for the limitation will be described. Finish rolling finish temperature (FT7)
The lower limit is set to 800 ° C. in order to prevent deterioration of characteristics due to appearance of a processed structure (processed ferrite) and a layered structure. A preferable range is 850 to 900 ° C.

Next, winding will be described. The lower limit of the winding temperature is 550 ° C. so that the second phase is softened and the micro Vickers hardness ratio of the ferrite and the second phase (second phase hardness / ferrite hardness) becomes 1.5 or less. Preferably, it is 575 ° C. or higher. The upper limit is not particularly defined, but is preferably 700 ° C. or less from the viewpoint of reducing scale loss and suppressing microstructure coarsening. Also, so-called quenching immediately after rolling or multi-stage cooling may be performed for the purpose of increasing the space factor of ferrite, refining the ferrite and the second phase, and further reducing the length of the hot run table.

The above is the reason for regulating the production method of the present invention. In order to increase the effect of increasing the space factor of ferrite and the effect of refining the ferrite and the second phase, the upper limit of the heating temperature is set to 115.
0 ° C, finish rolling start temperature (FT0) is 10
Means such as a temperature of not higher than 00 ° C. and a total reduction of 85% or more in the finish rolling may be performed alone or in combination.
However, when Nb is contained, the heating temperature needs to be 1000 ° C. or higher from the viewpoint of solid solution.

The steel slab to be rolled may be any of so-called cold flake reheating, HCR and HDR.
Also, a steel slab by so-called thin continuous casting may be used. Further, the hot-rolled steel sheet according to the present invention may be used as a plating base sheet, or the hot-rolled steel sheet according to the present invention may be manufactured in a thick steel plate manufacturing facility having no winding step.

[0025]

EXAMPLE A steel slab obtained by casting steel having the chemical composition shown in Table 1 was subjected to hot finish rolling, cooling, and winding to obtain a steel sheet. Table 2 shows the steel sheet microstructure, Table 3 shows the properties of the steel sheet, and Table 4 shows the method of manufacturing the steel sheet. Examples of the present invention are steels A to F. Comparative examples are G steel, H steel, and I steel.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

Assuming actual use, as a characteristic value to be satisfied by a steel sheet, TS × T · EL (strength−strength−
Total elongation balance) ≧ 18000, TS × U · EL (strength-uniform elongation balance) ≧ 12000, d / d0 (hole expansion ratio) ≧ 1.2, YR (yield ratio) <90%, fatigue characteristics, fatigue limit Ratio ≧ 0.50, ΔTS ≦ 5 for heat-resistant softening
5 MPa is required. Preferably, the transition temperature ≦ -40 ° C. is desired as the low-temperature toughness.

In the example of the present invention, a steel sheet satisfying simultaneously the three properties of workability, fatigue property and heat softening property is obtained, and the low temperature toughness is also good. On the other hand, in the comparative example, workability,
At least one or more of the fatigue characteristics and the heat-softening resistance has not been achieved. In addition, the example of this invention also showed favorable bending property, two-hour workability, and spot weldability. Furthermore, in the present invention, since there is no need to perform rapid cooling or low-temperature winding at the run-out table, material variations in the coil longitudinal direction and width direction are small.

The microstructure was evaluated by the following method. The particle size and space factor are determined by the Nital reagent and JP-A-59-2194.
No. 73, the cross section in the rolling direction of the steel sheet was corroded by a reagent (a mixed solution of picric acid, sodium thiosulfate, nitric acid, ethanol, and the like and a nital solution), and determined by an optical microscope photograph at a magnification of 1000 times. Hardness was determined by a micro Vickers test.

The characteristics were evaluated by the following methods. The tensile test was conducted according to JIS No. 5, and the tensile strength (TS), the yield strength (YP), the total elongation (T · EL), and the uniform elongation (U · E)
L) and local elongation (L · EL) were determined. ◎ In the hole expansion test, a 20 mm punched hole was pushed out from a burr-free surface with a 30-degree conical punch, and the hole diameter (d) when the crack penetrated the plate thickness and the initial hole diameter (d0, 20 mm) Expansion ratio (d /
d0) was determined. As the fatigue characteristics, a fatigue limit ratio (F) = 2,000,000 times fatigue strength / tensile strength was obtained by a swing plane bending fatigue test. The heat-softening resistance is as follows:
(Cooling after holding at 00 ° C for 5 minutes) and tensile strength before and after treatment (T
The change ΔTS in S) = TS before heat treatment−TS after heat treatment. The low-temperature toughness was measured using a 1/4 sub-size test piece with a 2 mm V notch, and the fracture surface transition temperature (vTrs) at which the brittle fracture ratio became 50% was determined.

The bendability was such that a test piece having a size of 35 mm × 70 mm was subjected to 90 ° V bending at a tip 0.5R with a burr on the outside, and cracks were observed. The secondary workability was such that a punched plate of 90 mmφ was cup-formed at a drawing ratio of 1.8, and was crushed at -50 ° C, and cracks were observed. The spot weldability was determined by observing the presence or absence of breakage in the nugget (the portion that was melted during spot welding and then solidified) when the spot weld test piece was peeled off with a chisel.

[0035]

According to the present invention, a hot-rolled high-strength steel sheet having unprecedented composite properties, that is, a hot-rolled high-strength steel sheet that simultaneously satisfies the three properties of workability, fatigue properties and heat-softening resistance is low-cost and stable. The use and conditions of use of the hot-rolled high-strength steel sheet have been greatly expanded, and the industrial and economic effects are extremely large.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-3679 (JP, A) JP-A-7-252592 (JP, A) JP-A-7-252584 (JP, A) JP-A-6-252584 65645 (JP, A) JP-A-5-195143 (JP, A) JP-A-5-59485 (JP, A) JP-A-57-181360 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] 1. A weight ratio of C: 0.03 to 0.20%, Al: 1.0 to 5.0%, Mn: 0.5 to 3.5%, Si ≦ 5.0%, P ≦. 0.05%, S ≦ 0.01% Contains the balance of Fe and unavoidable components, is composed of ferrite as a first phase and a second phase as a microstructure, and has a ferrite space factor of 50% or more, and Micro Vickers hardness ratio of the second phase (second phase hardness /
(Ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50. 2. C: 0.03 to 0.20% by weight, Al: 1.0 to 5.0%, Mn: 0.5 to 3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01% 800 ° C. using a steel slab containing the balance Fe and unavoidable components
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C. 3. C: 0.03-0.20% by weight, Al: 1.0-5.0%, Mn: 0.5-3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Ca: 0.0005 to 0.01%, or REM: 0.005 to 0.05% The balance contains Fe and unavoidable components. It is composed of a certain ferrite and a second phase, the ferrite space factor is 50% or more, and the micro Vickers hardness ratio of the ferrite and the second phase (second phase hardness /
(Ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50. 4. C: 0.03-0.20% by weight, Al: 1.0-5.0%, Mn: 0.5-3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Ca: 0.0005 to 0.01%, or REM: 0.005 to 0.05% Using a steel slab containing the balance Fe and unavoidable components, 800 ° C.
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C. 5. C: 0.03 to 0.20% by weight, Al: 1.0 to 5.0%, Mn: 0.5 to 3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Nb: 0.005 to 0.050% Contains the balance of Fe and unavoidable components, and is composed of ferrite as the first phase and second phase as a microstructure. The product moment is 50% or more, and the micro Vickers hardness ratio between the ferrite and the second phase (second phase hardness /
(Ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50. 6. A weight ratio of C: 0.03 to 0.20%, Al: 1.0 to 5.0%, Mn: 0.5 to 3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Nb: 0.005 to 0.050% Using a steel slab containing the balance Fe and unavoidable components, 800 ° C.
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C. 7. C: 0.03-0.20% by weight, Al: 1.0-5.0%, Mn: 0.5-3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Nb: 0.005 to 0.050%, Ca: 0.0005 to 0.01% or REM: 0.005 to 0.05% Inevitable with the balance Fe The microstructure is composed of the first phase ferrite and the second phase, and the ferrite space factor is 50% or more, and the micro Vickers hardness ratio of the ferrite and the second phase (second phase hardness /
(Ferrite hardness) is 1.5 or less, and as characteristics, tensile strength (TS) ≧ 400 MPa, strength-total elongation balance (tensile strength × total elongation) ≧ 1800
0 (MPa ·%) Strength-uniform elongation balance (tensile strength × uniform elongation) ≧ 12
000 (MPa ·%) A hot-rolled high-strength steel sheet for processing having excellent heat-resistance and softening properties, characterized by having a fatigue limit ratio ≧ 0.50. 8. A weight ratio of C: 0.03 to 0.20%, Al: 1.0 to 5.0%, Mn: 0.5 to 3.5%, Si ≦ 5.0%, P ≦ 0.05%, S ≦ 0.01%, Nb: 0.005 to 0.050%, Ca: 0.0005 to 0.01% or REM: 0.005 to 0.05% Inevitable with the balance Fe 800 ° C using a steel slab containing
A method for producing a hot-rolled high-strength steel sheet for processing having excellent heat-resistant softening properties, wherein hot-rolling is carried out at a temperature of from 1000 to 1000 ° C and winding is carried out at a temperature of from 550 to 700 ° C.